1. Field of the Invention
This invention relates to imaging systems. In a primary application of the invention relates to the processing of multicomponent x-ray and magnetic resonance images.
2. Description of Prior Art
In many branches of imaging an array of image measurements are combined to provide a desired selective image. This is particularly true in medical imaging where x-ray or magnetic resonance measurements can be combined to emphasize or cancel specific body materials, significantly improving the diagnosis. This process is described in a paper by A. Macovski in the IEEE Trans. on Medical Imaging., MI-1: 42 (1982), entitled "Selective Projection Imaging: Application to Radiography and NMR."
A problem with this selective operation is the reduced SNR. The elimination of certain materials requires that negative weights by applied to some of the measurements. This reduces the mean values, while the variances add, thus reducing the SNR. This problem was significantly improved by the system of U.S. Pat. No. 4,463,375 issued to Albert Macovski. Here the measurements are combined to provide both the desired selective image, and a nonselective low-noise image where the weights are all positive. The final processed selective image is then derived from the low-frequency portion of the selective image and high frequency information from the low-noise image. In the latter operation, care must be taken that only the desired high frequency information, of the correct amplitude and polarity, is chosen. In the patent cited, one method of providing the correct high frequency information was to weight the high frequency image by the ratio of the derivatives of the selective and low-noise signals. This, however, produces some instabilities and can require the use of threshold devices to limit the range of operation.
As indicated in U.S. Pat. No. 4,463,375, a primary application is dualenergy radiography and computerized tomography. These systems can also be applied to multiple component NMR images including cross-sectional images and selective projection images as described in U.S. Pat. No. 4,486,708.